The proposed investigation will examine the effects of a comprehensive school-based physical activity intervention on physical activity, physical fitness, and psychosocial determinants of physical activity in high school girls. It is known that physical activity levels are lower in girls than in boys and that physical activity declines in both genders during adolescence. The intervention, which will be targeted at reducing the rate of decline in physical activity in girls, will include modifications of physical education, health education, school health services, and school environment. These modifications will be designed to provide girls with positive physical activity experiences though participation in physical activities that are culturally relevant and to provide enhanced social and environmental support for physical activity. A nested cohort experimental design will be used, and school will be the primary unit of analysis. Subjects will be girls in twenty-four secondary schools. Schools will be matched on demographic characteristics and randomly assigned in equal numbers to intervention or control groups. Baseline testing will be performed during spring of the subjects' eighth grade year with follow-up tests being administered one and two years later. Dependent variables will include self-reported participation in moderate and vigorous physical activity in and outside of school, physical fitness measured as cardiorespiratory fitness and muscular strength/endurance, and psychosocial determinants of physical activity including self-efficacy, social influences, beliefs, and intention. This study will provide important information concerning the effectiveness of an intervention that addresses important national health objectives and that would be readily generalizable to secondary schools throughout the nation.